Caption: Arctic icebreaker. Icebreakers have reinforced hulls and are used to break through sea ice and open up passageways for other ships. This is the nuclear-powered Russian icebreaker Rossiya. It has two nuclear reactors: one is used to power the ship, and the other is the backup energy supply. Nuclear-powered icebreakers can only travel in cold waters, in order to cool their reactors. Photographed off Murmansk in the Russian Arctic in 2002.